The early phases of blood coagulation involve factor XII, factor XI, prekallikrein, kininogen, and perhaps other proteins. During the past several years, we have extensively purified factor XII and factor XI in order to study the interaction of these various inactive precursor proteins in the early phases of blood coagulation. Initially, the activation of factor XII and factor XI will be studied employing proteases, such as trypsin, to gain insight as to the mechanism of activation for these proteins. Similar studies will then be carried out in the presence of high molecular weight kininogen and prekallikrein in order to clarify the mechanism of activation of factor XII and factor XI under more physiological conditions. The major goal in each case will be to establish what specific peptide bonds are cleaved during the activation reaction. Studies will be carried out to examine the mechanism by which plasma inhibitors, such as antithrombin III, inhibit the coagulation process. Specifically, these studies will focus initially on factor Xa in order to establish, if possible, the nature of the complex formed between antithrobin III and factor Xa. Specific efforts will attempt to focus on the residues in antithrombin III involved in this reaction. Another goal of this research program is to examine the activation mechanism of human factor IX and human factor X which have been extensively purified in our laboratory. In these studies, the two coagulation factors will be activated with the purified protease from Russell's viper venom. Hopefully, the specific bonds that are cleaved during the activaton reaction for these two proteins will be established. BIBLIOGRAPHIC REFERENCES: Kurachi, K., Schmer, G., Hermodson, M.A., Teller, D.C., and Davie, E.W., Characterization of Human, Bovine, and Horse Antithrombin III, Biochemistry 15, 368 (1976). Kurachi, K., Fujikawa, K., Schmer, G., and Davie, E.W., Inhibition of Bovine Factor IXa and Factor Xa beta by Antithrombin III, Biochemistry 15, 373 (1976).